In order to recognize meaningful prognostic biomarkers of TNBC, this research analyzed two profile data sets (GSE36693 and GSE 65216) using bioinformatics methods. A total of 76 TNBC specimens and 175 non-TNBC specimens were included in the current research. Through the online tool analysis of GEO2R and Venn diagram, we found 140 common DEGs in total (|logFC|>2 and adjust P < 0.01). Then, the results of GO and KEGG Pathway Enrichment Analysis utilizing DAVID online tool were as followed: 1) when mentioning to biological processes (BP), up-regulated DEGs were significantly enriched in peripheral nervous system development, epidermis development, single organismal cell-cell adhesion, positive regulation of transcription from RNA polymerase II promoter, cytoskeleton organization etc., while down-regulated DEGs were enriched in phosphatidylinositol three-kinase signaling, positive regulation of transcription from RNA polymerase II promoter, regulation of intracellular transport, wound healing and so on; 2) when it comes to molecular function (MF), up-regulated DEGs were particularly enriched in transcription factor activity, RNA polymerase II distal enhancer sequence-specific binding, chitinase activity, chitin binding, and down-regulated DEGs were significantly enriched in RNA polymerase II transcription factor binding, calcium ion binding, estrogen response element binding, dystroglycan binding, transcription regulatory region DNA binding; 3) for cell component (CC), up-regulated DEGs were enriched in the extracellular space, extracellular exosome, epidermal lamellar body, intermediate filament while down-regulated DEGs were significantly enriched in extracellular space. For KEGG pathway analysis, DEGs were obviously enriched in p53 signaling pathway, prostate cancer and metabolic pathways (P < 0.05). Then, by using STRING online tool as well as Cytoscape, we constructed DEGs PPI network complex composed of 94 nodes and 180 edges. In addition, 29 vital genes were screened from the PPI network complex via the analysis of Cytoscape's MCODE. Next, we performed KEGG enrichment analysis again on 29 DEGs through DAVID and found that these genes were enriched in the p53 signaling pathway, pathways in cancer, oocyte meiosis and prostate cancer (P < 0.05). After taking the results of both PPI analysis and the KEGG pathway enrichment into consideration, we found that CCNE1, CDKN2A, AR, SERPINB5 and IGF1 among the 29 selected genes could play a key role in common significantly enriched pathways. The 5 genes were verified by GEPIA analysis, and it was found that compared with non-TNBC samples, 3 genes showed high expression in TNBC samples (P < 0.05). Finally, through cBioportal, we found that only CCNE1 had a significantly worse survival, which could be used as a new potential target for providing new treatment ideas for TNBC and improving patient prognosis..
CCNE1, also known as cyclin E1, is encoded by this gene belongs to the highly conserved cyclin family. The characteristic of its members is that the protein concentration changes drastically with the cycle throughout the cell cycle. Cyclin is a regulator of cyclin-dependent kinase (CDK), CCNE1 forms a complex with CDK2 as a regulatory subunit whose activity is necessary for cell cycle G1/S transition [21]. The protein is abundantly present at the boundary of G1-S phase and degraded as the cell cycle passes through S phase. This gene has been observed to be highly expressed in many tumors, which can lead to chromosomal instability and contribute to tumorigenesis. The dysregulation of CCNE1-CDK2 activity is related to a variety of cancers including nasopharyngeal carcinoma, bladder cancer and breast cancer, and has been fully proven [22–24]. Accumulating data proved that TNBC frequently expressed CCNE1, while ER positive cancer did not [25], and the absence of CCNE1 for poorer DFS [25]. In the p53 pathway, p53 acted as a tumor suppressor gene. But Contrary to the activation of p53 regulatory checkpoint or apoptosis, the expression of cyclin E protein promotes the process of entering S phase from G1 phase. And the lack of p53 function gives tumor cells an escape gap, so that tumor cells can avoid cell cycle arrest or cell death, and advance to the next stage through this disorder and uncontrolled growth [21, 26]. In addition, the loss of functional expression of the G1 checkpoint CDK inhibitor, p21 (CDKN1A) is also related to the carcinogenesis and disease progression of breast cancer, at the same time, more and more data indicate that the loss of function of p21 can mediate the drug-resistant phenotype which always means a poor prognosis [27]. In our bioinformatics analysis, the phenomenon of the up-regulated CCNE1 enriched in the p53 pathway was verified again and it CCNE1 really plays an important role in TNBC, but relative clinical practice is lacking.
As for pathways in cancer, specifically in the cell cycle, the transcription factor E2F1 and the tumor suppressor protein retinoblastoma (RB) are two key factors that regulate the progression of the cell cycle. They determine whether the cell can carry out the process of DNA replication and cell division by regulating the checkpoints of G1/S and G2/M together [28]. The CCNE1/CDK2 complex can phosphorylate RB, then release E2F1 and activate its transcriptional activity to advance the cell cycle from G1 to S phase, while dephosphorylation of RB promotes E2F1 heterodimerization while inhibiting E2F1 activity [29, 30]. It can be seen in the Fig. 7B that Cyclin E-CDK2 is associated with RB gene in the process of pathways in cancer, and this enrichment of CCNE1 was validated by us as well. There is no doubt that CCNE1 can be a useful target of TNBC in the future.
Numerous studies have proved that CCNE1 was related to the carcinogenesis and development of various types of cancer. Nevertheless, researches and clinical practice reported about this gene in TNBC is insufficient, in other words, they haven’t been taken seriously enough. Further experiments should be carried out on CCNE1 to better validate its functions. Thus, results in our research may provide useful information and directions for prospective research on TNBC treatment and prognosis. But this needs to be further verified through experiments in vitro or in vivo.